But you can do "canceling" which makes the problem easier
notice that
\[ \frac{10}{21} \cdot \frac{3}{5} = \frac{10\cdot 3}{21 \cdot 5} \]
but you know (right?) that 10 * 3 is the same as 3*10, so you could write this as
\[ \frac{3\cdot 10}{21 \cdot 5} = \frac{3}{21} \cdot \frac{10}{5}\]
you can divide top and bottom by 3 to simplify 3/21 to 1/7 and simplify 10/5= 2
so the problem is
\[ \frac{1}{7}\cdot \frac{2}{1} = \frac{2}{7} \]